The present invention relates to a procedure for arranging water circulations in an integrated paper mill, including a pulp mill based on refiner mechanized pulp and/or groundwood pulp and/or waste paper and/or chemical pulp, and paper and/or board manufacturing lines.
Various impurities enter the water circulations in a paper mill in different phases of the pulp and paper production process. Fouling of water is mainly caused by organic ingredients accumulating from fiber raw material and, on the other hand, mainly by inorganic chemicals added therein in different phases. As unwanted chemical agents, such impurities, in high proportions, are detrimental to the operation of the process and the quality of the paper produced.
A paper making process can be divided into two or three main phases being partly separate in water circulations, the first phase thereof including pretreatment, defibering and cleaning of fibre raw material and preferably also pulp thickening and pressing. The second phase includes improving of pulp quality, and paper manufacture. Currently, also the water circulation of the paper manufacture has been separated, whenever needed, from the water circulation of the quality improvement phase into a third water circulation by arranging pulp thickening and/or pressing between the process phases. The second quality improvement phase includes, depending on the need, pulp processing phases such as dispersing, bleaching and sorting.
The pretreatment may include various operations, such as barking, washing of chip, thermal and chemical treatment, waste paper pulping and pulp cleaning.
In pretreating raw material, namely in the defibering, pulping and cleaning phases, addition of chemicals can be employed, or the phases can be carried out without any chemicals. When producing mechanical pulp, about 2 to 5 per cent of the wood material is dissolved or dispersed as colloid particles in the process water. Most of the process water fouling takes place in connection with chip treatment and defibering, whereby, for instance, the water coming from the mass suspension of a TMP grinder contains dissolved and colloid organic matter in great quantities. Moreover, when using waste paper, paper fillers and additive agents from the raw material end up into the circulation water in the defibering and cleaning phases, in addition to organic agents, additive agents used in converting paper, and completely foreign agents accumulated in the use or recycling of paper. Particularly difficult foreign agents are sticky agents which are adhesive in nature and originated from various adhesives and plastics.
Inorganic chemicals enter the water circulation in the pulp quality improvement phase, in bleaching and in the wet end of the paper machine, in which phases also organic matter is dissolved. In addition, in different phases of producing recycled mass, chemicals are used in great quantities e.g. for chalking print dye pigments, as auxiliary foaming agents, in pulp bleaching, and in end acidification.
In different phases of a paper making process, great quantities of water are needed for various purposes, such as for dilution of pulp and chemicals, transport and cleaning. In addition, water is needed e.g. for cooling and sealing. By circulating the process waters of the paper mill, endeavors are made to recover and to reuse the useful ingredients originated from the process and contained in the circulation water, such as fibers, fines, filling agents and paper making chemicals, as well as heat. Some of the water needed by the paper mill is generally provided from outside the mill as raw water which has to be cleaned prior to introduction to use and heated if need be to appropriate temperature. Clean water is needed e.g. for certain washing jets in the paper machine and for dilution of chemicals.
In order to maintain the contents of unwanted agents below the risk limits, fresh water is usually brought into the process at the same time as part of the circulation water is removed from the system as effluent. When the paper mill process waters are generally circulated counter-current from the paper machine towards the pulp mill, the organic and inorganic agents entered in the circulation water in different phases of the process are usually concentrated most in the circulation and effluent waters of the defibering phase of the pulp. On the other hand, the organic and inorganic matter released in different process phases travels to some extent downstream together with the pulp to the paper machine, which travelling is attempted to be reduced by the thickening, dilution and pressing apparatus positioned between the pretreatment, defibering and cleaning phases and the pulp quality improvement phase, which systems have currently been positioned, whenever needed, also between the pulp quality improvement phase and the paper making phase. With the aid of presses, the pulp can be thickened, depending on the apparatus, to about 25 to 35% thickening, whereas the thickening achieved with the aid of thickening apparatus is only about half thereof.
In Finnish patent applications Nos. 962176, 962177, 062178, an arrangement is described in which some of the fresh water amount usually needed by the paper mill is replaced by jet waters selectively recovered from the waters fouled in the paper mill and by waters to be cleaned locally, and the concentrates thus obtained from the water cleaning phases are utilized whenever appropriate by making them flow counter-current relative to the raw material flow for reuse in the paper mill, by means of which measures the level of the quantities of unwanted agents circulating in the paper mill is controlled. By said arrangement, unwanted agents are transferred counter-current from the water circulation of the paper machine into the water circulation of the pulp mill, wherefrom they can be conducted to an effluent evaporation plant and concentrated for combustion at the same time as the cooling water circulation of the evaporation plant is advantageously connected to the cooling and jet water system of the paper mill and the clean condensate is conducted as jet water for substitution of fresh water.
The organic and inorganic matter circulates in due time in the water circulations of the mill and is finally discharged from the process mainly among the effluents and to a lesser extent with the paper and into the atmospheric air. The main fractions of the effluents are formed by the controlled overflow of the filtrate of the circulation water system of the pretreatment /defibering /cleaning phases and by the reject waters of the cleanings of the pulp and the water circulations in different phases, in addition to which sporadic emissions occur to some extent.
Pressure towards reducing the consumption of fresh water by closing water circulations are on one hand caused by the costs of raw water and the effluent, and on the other hand, by the availability of raw water and by the emission restrictions concerning effluents. A complete shut-off of a paper mill is not possible because emissions of effluents are needed for removal of unwanted agents from the process. When reducing the consumption of the fresh water entering into the water circulation of the paper machine mainly through jets and dilution targets, an excessive temperature rise and concentration of colloid and dissolved organic and inorganic unwanted agents, particularly salts, in the water circulations of the paper machine and of the pulp mill may become a problem to the extent which is detrimental to the quality and production of pulp and paper. If e.g. the content of unwanted agents in the circulation water of the paper machine is increased too much, the action of the retention chemicals will be weakened, the formation of interfiber bonds will be impaired and the strength of paper being manufactured will suffer. A first drawback lies therein, that for instance with the increased content of the agents dissolved in the circulation water used for pulp bleaching, the degree of brightness may drop or the consumption of bleaching chemicals may grow, which, in turn, will increase the amount of organic and inorganic matter ending up in the circulation and effluent waters.
With the most commonly used water purification methods within the paper mill, fibre, pigment, resin or sticky agent particles are in general removed using filtering, clarification and flotation techniques. However, removal of unwanted colloid agents is incomplete, and separation of dissolved agents hardly ever occurs, instead, the agents travel into different fractions mainly in proportion of liquid flows. A more thorough removal of colloid matter and dissolved organic and inorganic matter requires costly special arrangements such as membrane filtering and/or evaporation.
By evaporation, biologically undecomposable organic matter, micromolecular agents and volatile components and salts can be removed from the unclean water, the separation whereof would otherwise be difficult. The water achieved through evaporation may be even purer than the chemically treated fresh water. The use of evaporation as a process-internal water cleaning method is particularly advantageous when the waters being cleaned are warm and concentrated, and the process yields waste heat in great quantities appropriate for use as an energy source. Utilization of waste heat can advantageously be intensified by means of surface condensation and cooling tower connection of the evaporation as disclosed in FI patent application No. 962178. When the filtrate of the thickening phase subsequent to the defibering is strong and hot, owing to the counter-current connection of the circulation waters, and its quantity is small, it is advantageous to lead it to the evaporation plant to be concentrated further. From the evaporation plant the concentrated solution can be conducted to the combustion unit, whereby the organic matter is utilized in the energy production of the paper mill and the inorganic matter, such as sodium and sulphur, in the chemical circulation of the mill. The foul condensates of the evaporation plant can be conducted to biological effluent treatment.
In manufacturing mechanical pulp, great quantities of expansion steams are formed, for instance, in making refiner mechanical pulp, about 2 tons of steam are released per pulp ton. Another drawback lies therein, that merely 1 ton of pure steam per mass ton can be produced with expansion steam with the aid of a steam generator, said steam being used in the process for replacing fresh steam. The rest of the thermal energy of the expansion steam is wasted in heat losses and discharged with the unclean condensate into the circulation waters and effluents.
In a paper mill provided with conventional counter-current connection of circulation waters, the effluents contain great quantities of inorganic salts, a majority of which being compounds melting at a low temperature, such as sodium. Another drawback in this case is that an effluent concentrate cannot be burnt together with mill waste or other fuel in the ancillary boiler available which can be of grate type, or nowadays most frequently, fluidized bed or circulated type, in which sodium and potassium cause sintration of bed sand and chloride, boiler corrosion. Yet another drawback is that burning such concentrate requires a soda boiler which may not always be available in conjunction with the mill. Another drawback is that in such instances detrimental and corroding unwanted agents, such as potassium and chloride, may be concentrated into the chemical circulation of the pulp mill. When the bleaching waters of pulp mills are concentrated and conducted into a soda recovery boiler, endeavors are made to reduce concentration of said agents by means of crystallization techniques and by opening chemical and water circulations. Another drawback is that the evaporation concentrate has a low dry matter content (20 to 40%), and decreases the efficiency of the electricity and steam production of the high-pressure ancillary boiler or soda recovery boiler.
The object of the present invention is to achieve a procedure, with the aid of which the above drawbacks can be avoided and simultaneously, the water consumption of the paper mill and effluent emissions can be reduced.
Another object the present invention is to achieve a procedure with which the combustion of agents ending up in the concentrate of the effluent evaporation plant, utilization of combustion waste or the amount of agents detrimental to recovery of chemicals can be limited.
Yet another object of the present invention is to achieve a procedure with the aid of which the contents of colloid and dissolved organic and inorganic unwanted agents can be reduced in the water circulations of the pulp quality improvement phases and of the paper machine when reducing the fresh water consumption of the mill.
A further object of the present invention is to provide for a more efficient utilization of the heat of expansion steams of the process and the condensate and energy contents of effluents.
In the procedure according to the present invention, the circulation water of the water circulation of the first process phase is concentrated by circulating the filtrate of the pulp thickening phase after the defibering and cleaning phases back into the pretreatment and defibering of the raw fiber material. The concentration of the circulation water is intensified by adding the dilution and pressing of the pulp after the pulp thickening phase after the cleaning. In addition, a second pulp dilution/press phase can be added in the process immediately after the defibering. The water circulation of the first process phase is separated into a water circulation of its own, within which the overflow connection between the water circulations of the thickening phases is arranged counter-current relative to the pulp flow so that the filtrate of the pressing passes into the pulp dilution on the front side of the thickening and the filtrate of the thickening goes into the dilution of the pulp prior to potential pressing after the defibering. The pretreatment of the raw material may comprise, for instance in chip treatment, removal of impurities and washing, absorption and heat treatment with the aid of water and steam and/or with the aid of chemicals, whereto the filtrate from the pressing and/or thickening is conducted. The pretreatment of raw material may also include other operations such as barking or waste paper pulpering and cleaning.
The strongest filtrate obtained as overflow of the pretreatment and from pulp pressing and/or thickening after the defibering phase is concentrated in the evaporation plant and the concentrate burnt.
The evaporation concentrate can be burnt in a fluidized or circulated bed type ancillary boiler when chemicals harmful to combustion or causing corrosion are not conducted into the first water circulation, nor the circulation waters including such agents are not conducted counter-current relative to the traveling of the pulp or from the water circulations of the paper machine or the quality improvement phase.
The evaporation concentrate can be burnt in a soda recovery boiler when a concentrate contains, due to the chemical selections of the process phases or the water connections, compounds melting at low temperature, such as sodium, but not in excessive quantities corrosion-causing chemicals or chemicals detrimental to recovery of chemicals and unwanted agents.
The evaporation concentrate can be dispersed into drops or burnt in a melt fuel furnace with auxiliary fuel when the concentrate contains compounds causing corrosion or melting at low temperature when it should not cause harm to the burning, chemical circulation or efficiency of power production. The inorganic molten salts obtained from the bottom of the furnace can be dissolved in residual water to be taken into biological purification or solidified into solid state to be taken to recycling or to a dump in separation or mixed with other waste.
For the energy source in the effluent evaporation plant, electricity, steam and preferably the hot, unclean waste steams of a refiner plant or a pressure grinding plant can be used, being conducted through steam wash directly into the evaporation phase. With the aid of steam wash, the fibre matter is prevented from entering on the heat-exchange surfaces of the evaporation plant. In this manner both the amount and the heat of expansion steams of the pulp production and the heat content of the effluent can be utilized more efficiently than before. The clean condensate formed from unclean steams and circulation water is used for pulp dilution prior to the press phase. The unclean condensate formed in cleaning the main condensate is conducted into the effluent treatment together with the reject fractions of the first process phase. The water quantity leaving the water circulation together with the pulp, entering into the effluent and the atmospheric air, is replaced by other waters free from said detrimental agents, such as by the circulation water of the paper manufacturing, but not by the bleaching water of the quality improvement phase containing sodium in great quantities. Preferably, clean cooling water heated in the cooling/condensing apparatus can be used for this purpose. Said make-up water together with the clean condensate is conducted into the pulp dilution prior to the pressing preceding the quality improvement phase, whereby the washing of pulp is carried out preferably with water which is as clean as possible.
The pulp can be properly washed, in this manner, before the bleaching or dispersing of the pulp quality improvement phase, which will improve the quality of pulp and paper, especially clearness and microbiological purity, reduce the quantities of detrimental agents transferring to the paper machine and the use of chemicals, and water emissions from the water circulations of the quality improvement phase and the paper machine. The overflow of the paper machine water circulation is conducted primarily into the dilution before the pressing of the pulp after the quality improvement phase. Between said phases, the counter-current principle can thus be maintained, so that the effluent formed therefrom is mainly comprised of controlled overflow of the filtrate of the quality improvement phase, reject waters of the phases, and spurious emissions.
From about 50% to a majority of the organic effluent load of the plant is dissolved in the first water circulation being separated as a separate water circulation. When said effluent is concentrated by evaporation into a small volume and the concentrate is burnt, the energy content of the organic matter can be recovered. Hereby, also the amount of the water to be discharged as effluent from the process is reduced, as well as the amount of agents difficult to decompose biologically or of those agents which will not decompose, so that the efficiency of the biological treatment is improved and problems related to sludge elimination are reduced.
The procedure of the invention is advantageously appropriate for use in an integrated paper mill in which the use of expansion steams of thermomechanical pulp and pressure groundwood can be intensified by conducting them into an effluent evaporation plant, but the procedure is also applicable in a mill utilizing waste paper as raw material, when the water circulation to be concentrated includes a pulp concentration phase after the cleaning, and the dilution and pressing, or, in addition thereto, as an advantageous embodiment, a second pulp dilution and pressing immediately after the pretreatment, pulpering and defibering phases. For the evaporation energy, electricity, fresh steam and thermal energy partly recoverable from flue gases can be used.
When the waste paper mass is washed according to the invention as early as in the first process phase on the counter-current principle two or three times, the ending up of impurities with the pulp in the paper machine is lessened radically. In the pulp thickening and/or pressing subsequent to the second, that is, the quality improvement process phase, the circulation water conducted from the third process phase, that is, the water as a controlled overflow from the paper machine, is advantageously used for the front dilution water. The discharge of impurities is also increased in that the water circulation of the pulp mill is in this manner divided into two parts in separation from each other, and the effluent of the second water circulation, that is, of the pulp improvement phase, is conducted into an external effluent treatment plant. When at this stage the counter-current principle is abandoned, the impurities ended up in the circulation waters in the second and the third phase of pulp production, such as sticky agents which are known to be very harmful in the production of recycled pulp and dispersed in the water in the dispersing phase, are not returned unpurified into the process, instead, they can be discharged efficiently.